The invention relates to a motor vehicle parking brake system, which is operable by non-muscular energy.
A motor vehicle is known from European patent document EP 0 478 642 B1, for example, in which an electric motor acts on the parking brake through a threaded spindle. Operation of the parking brake by non-muscular energy relieves the burden on the driver. In addition, convenience functions such as "stopping at a light", "starting assistance", etc. can be performed.
German patent document 195 16 639, moreover, describes a motor vehicle whose parking brake can be operated by a hydraulic pressure generator, for example a power brake controllable by non-muscular energy or an ASC (anti-slip control) pump.
The object of the invention is to provide an improved parking brake system of the generic type referred to above.
This goal is achieved by the parking brake system according to the invention, which is operable both by a hydraulic pressure generator and an electromechanical unit, which are selectively activated as a function of the operating state of the vehicle. The combination of hydraulic and electromechanical actuation of the parking brake according to the invention permits optimum use of the advantages of the two systems. The choice of operating mode or the switch between the two modes is preferably accomplished automatically, (without intervention by the driver), with one or more control devices that detect parameters of the vehicle, environment, driver's wishes, etc. acting accordingly on the hydraulic pressure generator or its valves or on the electromechanical control unit.
Hydraulic actuation permits rapid application and release of the parking brake (a few tenths of a second by comparison to one to two seconds with electromechanical operation) and is designed according to the system for a large number of load changes. Hydraulic actuation allows high braking pressures to be applied without difficulty, but results in relatively high cost (expense, weight, space requirements) for electromechanical control units.
The electromechanical control unit, assuming an intact on-board vehicle electrical system, can be used independently of the operating state of the vehicle.
In those vehicles that have a secondary brake circuit in addition to the primary service brake circuit, hydraulic actuation of the parking brake is possible with only a very slight additional expense. Such a secondary braking circuit serves for example for automatic slip control (ASC) or directional stability control (DSC) and uses as a pressure generator, for example, a power brake controllable by nonmuscular energy or a separate pump (ASC or DSC) pump. The electromechanical control unit as a rule consists of an electric motor with a self-locking transmission.
Parking with hydraulic actuation can be performed by both the brake shoes of the service brake and by separate parking brake shoes. The electromechanical control unit as a rule acts on the brake shoes which in turn act either on brake discs or brake drums of the service brake or on separate brake drums.
The combination of the two operating modes offers a number of functional advantages that will be explained in the following with reference to the possible embodiments of the invention. It is sufficient in this respect to design the electromechanical control unit for only a relatively small number of load changes, since the frequently engaged convenience functions (stopping at a light, starting assistance) can be handled by the hydraulic actuation particularly well. Of course the parking brake system must be designed so that the switch to the other actuating device takes place only when the system to which the switch is being made has developed a braking force that is at least as high as the braking force in the system from which the switch is being made.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.